1. [3H]H12-HTX binding to Torpedo electric organ membranes is saturable and inhibited by ligands that modulate EPCs. There is a small proportion of nonspecific binding to the membrane. 2. The validity of utilizing [3H]H12-HTX as a specific label for the ionic channel of the nicotinic receptor is extablished by the good correlation between the potency of HTX analogues in reducing EPC amplitudes and their inhibition of [3H]H12-HTX binding. 3. Receptor drugs and toxins inhibit [3H]ACh binding to the ACh receptor, but do not inhibit significantly [3h]h12-htx binding to the ionic channel at similar concentrations. 4. Various drugs and toxins with different modes of action modulate EPCs and inhibit [3H]H12-HTX binding to the ionic channel of the nicotinic receptor. Most such drugs and toxins, at similar concentrations, do not inhibit [3H]ACh binding to the ACh receptor, but some do, such as quinacrine and TEA. 5. The ionic channel of the nicotinic receptor is a protein which is solubilized by cholate and retains its affinity for H12-HTX as well as its drug sensitivity. 6. The ACh receptor is separated from its ionic channel, and different conditions affect the proportion of separated molecules.